1. Field of the Invention
The present invention relates to a semiconductor device used in the optical communication field and, in particular, to a semiconductor device provided with a high-NA (numerical aperture) lens in conformity with high-speed/large-capacitance optical communications.
2. Description of the Related Art
As shown in FIG. 7, in a conventional semiconductor device 50, an optical coupling construction is adopted in which light 52 radiated from the end surface of a light emitting element 11 is condensed on an end surface of an optical fiber 14 by using a ball lens 53.
In this semiconductor device 50, a silicon (Si) substrate 55 shown in FIGS. 8A through 8C is used.
In the silicon substrate 55, there is formed in the upper surface portion 55a thereof a V-shaped groove 55b having a substantially V-shaped (trapezoidal) sectional configuration. This V-shaped groove 55b is formed by performing anisotropic etching on the surface of the silicon substrate 55 by using a resist mask formed by photolithography.
And, in the silicon substrate 55, the edge portion connected to the upper surface portion 55a of the V-shaped groove 55b exhibited inclined surfaces 55e, 55f and 55g having peculiar inclination angles due to the silicon crystal structure (xcex81, xcex82 and xcex83, which are all 54.7 degrees).
And, in the silicon substrate 55 of the optical semiconductor device 50 shown in FIG. 8, the positioning of the light emitting element 11 is effected in the upper surface portion 55a near the V-shaped groove 55b, the positioning of the ball lens 53 being effected in the V-shaped groove 55b, the optical axes of the light emitting element 11 and the ball lens 53 coinciding with each other.
However, in the field of optical communications, there is an ever-increasing demand for increasing the communication speed and decreasing the optical coupling loss between the optical semiconductor device 50 constituting the optical coupling of the light emitting element 11 and the optical fiber 14. The optical coupling loss greatly influences the speed of the optical communication and may thus be an obstruction to high-speed optical communication.
In view of this, the present applicant has proposed use of an aspheric lens to decrease the optical coupling loss, instead of the ball lens 53.
As shown in FIGS. 9A and 9B, in the optical semiconductor device 60, instead of the conventional ball lens 53, an aspheric lens 63 is mounted and fixed in the V-shaped groove 55b of the silicon substrate 55.
As shown in FIG. 10, this aspheric lens 63 consists of a limited type lens of an optical glass and comprises a lens main body 63a provided with both-side convex aspheric surfaces, and an edge portion 63b in the peripheral edge of the lens main body 63a, the outer diameter (xcfx86) being 1.0 mm, the lens thickness (tc) being 0.81 mm, the optical length (L=L1+tc+L2) being 3.56 mm, the focal distance (L2) being approximately 2 mm, NA (numerical aperture) being 0.45, the magnification (m) being 3. Further, the distance (L1) from the object point to the apex of the lens surface being 0.3 mm.
Here, the NA can be generally expressed by the following equation.
NA=n sin xcex8
where xcex8 is the angle made by the ray having maximum opening of the rays emitted from the object point in the axis and the optical axis; and n is the refractive index of the medium where the object point exists. Thus, the larger the NA, the higher the resolution, making it possible to enhance the efficiency in optical coupling. Further, by making the lens in an aspheric configuration, it is possible to restrain the influence of the aberration.
In this way, in the optical semiconductor device 60 having the aspheric lens whose NA is 0.45, the output light 52 radiated from the end surface of the light emitting element 11 passes the aspheric lens 63 as shown in FIG. 9, and focuses on the end surface of the optical fiber 14 (See FIG. 7). This improvement decreases the loss in optical coupling as compared with the ball lens 53.
Incidentally, in this optical semiconductor device 60, to cope with the increase in speed and capacitance of optical communication and to utilize the characteristics of the aspheric lens to the utmost, it is necessary to further enhance the NA of the lens and reduce the WD (working distance=L1), which is the distance from the light emitting element 11 to the aspheric lens.
In the proposed optical semiconductor device 70 shown in FIG. 12, an aspheric lens 23 having high NA and short WD is mounted on a silicon substrate 55.
As shown in FIG. 11, the aspheric lens 23 consists of an infinite-type lens of optical glass and comprises a lens main body 23a provided with double convex aspheric surfaces and an edge portion 23b in the periphery of the lens main body 23a, the outer diameter (xcfx86) being 1.0 mm, the lens thickness (tc) being 0.81 mm, the focal distance (L2) being infinite, the NA (numerical aperture) being 0.60.
Generally speaking, in an aspheric lens, there is a strict demand for accuracy in optical axis matching as the NA increases. In this aspheric lens 23, the light output from one side becomes parallel rays, so that the optical axis matching can be conducted relatively easily.
However, as shown in FIG. 12, when the aspheric lens 23 having high NA is mounted as it is in the V-shaped groove 55b of the conventional silicon substrate 55 and fixed therein, a portion (H) is generated that interferes with the inclined surface 55g of the V-shaped groove 55b. 
Thus, there is a problem that the high NA aspheric lens 23 which utilizes the characteristics of an aspheric lens to the utmost and which has short WD cannot be mounted on the silicon substrate 55.
Further, as shown in FIG. 13, focusing attention on the outer diameter of the aspheric lens 23, it might be possible to prevent the generation of the above-mentioned portion H by reducing the outer diameter (xcfx86). However, from the viewpoint of the intention of maintaining high NA, it is necessary to further reduce the WD. As a result, the size of the aspheric lens 23 is only reduced in geometrical similarity, and, as the size of the lens is reduced, the WD is further shortened, making it impossible to prevent the generation of the portion (H) interfering with the inclined surface 55c. 
It is an object of the present invention to provide an optical semiconductor device of high NA having improved optical communication efficiency that increases the speed and capacitance in optical communication and allows mounting of a short WD lens.
As first means for solving at least one of the above problems, an optical semiconductor device is provided that comprises a semiconductor substrate having on one side an etched and substantially V-shaped first groove portion formed by etching, an optical element having an optical axis in the direction of the first groove portion and mounted to the one side, and a lens mounted in the first groove portion. The first groove portion comprises first and second opposing inclined surfaces and a third inclined surface perpendicular to the first and second inclined surfaces. A second groove portion is formed in the substrate and extends in a direction perpendicular to the direction of the first groove portion. The second groove portion includes the first, second and third inclined surfaces. The lens is mounted to the first and second inclined surfaces and has a part thereof protruding in the second groove portion. The optical element optically communicates through the lens.
Further, in the optical semiconductor device the second groove portion may be formed as a recess extending across the substrate.
Further, in the optical semiconductor device an edge portion of the lens may abut the side wall of the second groove portion.
A second means for solving at least one of the above problems includes a method of increasing coupling efficiency between an optical element, which is disposed on a surface of a silicon substrate, and an optical fiber. The method comprises positioning a lens disposed in an etched and substantially V-shaped first groove portion of the substrate having a first groove direction and having first and second opposing inclined surfaces and a third inclined surface perpendicular to the first and second inclined surfaces such that a part of the lens protrudes in a second groove portion extending in a direction perpendicular to the first groove direction and including the first, second, and third inclined surfaces.
A third means for solving at least one of the above problems includes a method of producing an optical semiconductor device that increases the coupling efficiency between an optical element, which is disposed on a surface of a silicon substrate, and an optical fiber. The method comprises etching first and second opposing inclined surfaces and a third inclined surface perpendicular to the first and second inclined surfaces to form a substantially V-shaped first groove portion in the substrate. The method also comprises forming a second groove portion in the substrate such that the second groove portion extends in a direction perpendicular to a direction of the first groove portion and includes the first, second, and third inclined surfaces.